A mounting arrangement of this type is disclosed in U.S. Pat. No. 4,392,640. According to this patent, the elastomeric elements between the wedge-shaped metal member and the frame-like metal member are arranged in such a manner that their effective working axes coincide.
Experience has shown that when traveling on certain types of road surfaces, motor vehicles (both of the front and rear wheel drive type) if equipped with the type of engine mounts disclosed in the above-mentioned patent can develop vibrations that are disconcerting to the vehicle occupants. Such vibrations are encountered primarily when the vehicle is traveling on a road surface having a harmonic profile, as in the case of concrete slabs with cross joints. Under these conditions, the drive unit mounted on the elastomeric elements is induced to harmonic oscillations in the frequency range between 8 and 15 Hz. Based on the mass and rigidity characteristics, the inherent frequency of a motor vehicle drive unit lies in this same range. If this inherent frequency range of the drive unit happens to coincide with the maximum of the road surface excitation, excessive vertical engine movements or accelerations encounter in this speed range. These accelerations are transmitted to the vehicle body and can have an adverse effect on the riding comfort.
One approach to eliminating vertical engine vibrations of the type described above is to use relatively hard engine mounts, i.e. elastomeric elements that exhibit a high degree of rigidity or stifffness. This will shorten the vibratory travel distances of the drive unit and, as a result, will shift the resonance frequency of the engine mount to higher frequencies and out of the maximum of the road excitations. Another relatively simple approach to rectifying the problem is the use of engine mounts with damping means at the elastomeric elements. However, since the elastomeric elements are also expected to isolate the second order ignition frequencies generated by the engine which, in the case of a four-cylinder engine running at 1000 to 6000 rpm, are in the range of 33 to 200 Hz, these chosen palliatives do not constitute an adequate solution, because to isolate second order ignition frequencies requires the use of soft elastomeric elements that have a low dynamic spring rate. The use of elements with damping characteristics also causes a marked increase in the dynamic spring rate.
Through tests one can demonstrate good acoustic results by using an elastomer which may be described by the following characteristics:
static inward deflection .eta.m=3.05 mm
at a static load F.sub.m =600 N (Newton),
dynamic spring rate K=460 N/mm
at an amplitude of the dynamic inward deflection 1o=.+-.0.5 mm
and a loss angle .delta.=3.4.degree.
at a frequency of 15 Hz.
However, a substantial deterioration of the acoustic results will already occur by using an elastomer having characteristics as defined as follows:
static inward deflection .eta.m=2.9 mm
at the same static load F.sub.m,
dynamic spring rate K=494 N/mm
at the same amplitude of dynamic inward deflection 1.sub.o,
loss angle .delta.=7.degree.
at the same frequency.
It is apparent from these two test results that the use of the known high damping type elastomeric grades in the drive unit mounts leads to a problematic acoustic increase of the dynamic soring rate K while, at the same time, an improvement can be realized in the low-frequency vibratory problem area. The increase of the loss angle .delta. by 108% corresponds to an increase of the dynamic spring rate of 20%.